1. Field of the Invention
This invention in general relates to photographic optical systems and in particular to a zone focusing optical system suitable for use in a camera of the type which includes an automatic ranging and focusing arrangement.
2. Description of the Prior Art
According to the well-known Gaussion lens equation, an image of an object which is located ahead of an optical system will only be sharply formed on film located behind the optical system when the distance between the optical system and the film plane of the system is correct according to the equation. Since the location of film in photographic cameras is typically fixed, this means that maximum image sharpness for any given object distance requires a unique focal length or a unique optical system to film distance. The process of adjusting the optical system focal length, or the optical system to film distance, to achieve image sharpness for different object distances is known as focusing and may be accomplished in well-known ways. One way of focusing, for example, is to provide a lens of fixed focal length in combination with a means for adjusting the spacing between the lens and film as a function of object distance. The usual arrangements for accomplishing this involve the use of a flexible bellows connecting the lens mount to the film retaining housing and a linkage for extending the bellows to alter the spacing between the optical system and film.
Another known focusing arrangement utilizes a variable focus optical system fixed in place ahead of the film. Here, the lens focal length is changed with object distance by changing the air spacing between individual elements of the lens by usually moving a front cell of the lens with respect to its other elements.
Another known focusing arrangement suitable for use with fixed lens-to-film distance situations involves the use of a rotatable lens turret which carries a plurality of lenses having different focal lengths each of which can be rotated into place along the camera taking path. These arrangements can be classified as zone focusing systems because they do not provide a continuously variable focal length but rather provide discrete focal lengths which sharply image for particular object distances and more or less sharply image objects located on either side of that object distance most appropriate for the particular lens focal length. In this manner, such systems provide adequate sharpness over a range or "zone" of object distances relying on the assumption that for any given focal length there exists a blur, due to defocus, of small enough size such that the performance of the system will not be adversely affected provided that the object remains within the appropriate zone. An example of such a system is shown and described in, for instance, U.S. Pat. No. 3,418,908 issued to E. H. Land on Dec. 31, 1968 and entitled "Range Finding-Focusing Apparatus For A Photographic Camera".
It is also known to provide zone focusing arrangement by combining a fixed lens with a turret having lenses which can be selectively indexed into alignment with the fixed lens such that the combination of the fixed lens with individual turret lenses operates to provide a series of different focal lengths that can be selectively changed. One such arrangement, which utilizes a fixed doublet, is described in U.S. Pat. No. 494,128 issued to E. Decker on Mar. 28, 1893 and entitled "Lens For Cameras".
Of the three focusing arrangements described, the first two have the advantage of continuous adjustability over a range of object distances. However, for a particular application this advantage must be evaluated in view of the increased cost associated with the need for bellows extension arrangements or more complex multi-element lens systems. Zone focusing systems, while not offering a continuous adjustability feature, are often adequate and offer an attractive alternate because they can in general be made inexpensively. However, most known zone focusing systems are somewhat cumbersome requiring a large amount of space because of their turret arrangements. Therefore, it is a primary object of the present invention to provide a zone focusing system which is inexpensive, compact and easy to manufacture.
It is another object of this invention to provide a simple zone focusing optical system that is favorably corrected to provide a usable image over a reasonable field and numerical aperture. As will be seen in the description that follows, this is done in part with meniscus lens elements which have aspheric surfaces for providing aberration control. Although it is known to use aspheric surfaces for this purpose, as disclosed, for example, in U.S. Pat. No. 3,902,792 issued to W. T. Plummer on Sept. 2, 1975 and entitled "Landscape Lens" and U.S. Pat. No. 4,105,308 issued to R. C. Owen et al. on Aug. 8, 1978 and entitled "Aspheric Plastic Triplet", the use of aspheric surfaces as described herein is believed to be novel.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed description.